1. Field of the Invention
The present invention relates generally to an apparatus and method for scheduling resources in a multiantenna system, and in particular, to an apparatus and method for scheduling resources for individual users in a multiantenna system.
2. Description of the Related Art
In general, the wireless channel environment, unlike the wire channel environment, exhibits low reliability due to multipath interference, shadowing, propagation attenuation, time-varying noise, interference, etc. this low reliability prevents the mobile communication from increasing its data rate. Many technologies for solving this problem have been developed. Two typical technologies are an error-control coding technique for controlling influences of signal distortion and noise, and a diversity technique for coping with fading.
The diversity technique copes with the fading by receiving several signals that underwent independent fading. Commonly, methods for obtaining the diversity effect include time diversity, frequency diversity, multipath diversity, and spatial diversity.
The time diversity, by being combined with channel coding and interleaving, obtains a time diversity gain. The frequency diversity, in which signals transmitted at different frequencies undergo different multipath signals, obtains a frequency diversity gain. The multipath diversity obtains a multipath diversity gain by separating multipath signals using different fading information. The spatial diversity, which uses several antennas for one of both or a transmitter and a receiver, obtains a spatial diversity gain by independent fading signals. The spatial diversity uses an antenna array.
A system using the antenna array (hereinafter referred to as an “antenna array system”), in which multiple antennas are included in a transmitter/receiver, aims at increasing frequency efficiency using spatial regions. The use of the spatial regions, compared with the use of the limited time regions and frequency regions, can obtain a higher data rate. The antenna array system is also known as a multiantenna system. Antennas constituting the antenna array system each transmit their independent information. Fundamentally, therefore, the antenna array system corresponds to a Multi Input Multi Output (MIMO) system.
Since the antenna array system has high frequency efficiency, in order to extend the system capacity, a correlation coefficient formed between transmission antennas and reception antennas should be low. When operating at the low inter-channel correlation coefficient, the information transmitted from the respective transmission antennas transmit over different channels, so a mobile station can distinguish the transmitted information. That is, the signals sent from the respective transmission antennas should have different spatial characteristics in order to be distinguished, contributing to extension of the channel capacity. In addition, the antenna array system is suitable for places where many multipath signals having different spatial characteristics. However, in a Line-of-Sight (LOS) environment, the antenna array system is not superior to the single-transmission/reception antenna system in terms of the increase in capacity. The antenna array system is suitable for environments where there are many multiple paths because of many scattering objects between a transmitter and a receiver, and especially suitable for the environment in which transmission/reception antenna channels have low correlation coefficients, i.e., the environment having the diversity effect.
In the antenna array system, a receiver provides channel state information to a transmitter. The channel state information needed in the antenna array system includes channel responses between the transmission antennas and the reception antennas. Therefore, the amount of the channel state information is proportional to the number of transmission/reception antennas.
The transmitter of the antenna array system requires scheduling of the resources to be allocated to individual users in order to maximize efficient use of the limited resources. The scheduling of resource allocation is achieved based on the channel state information provided from the receiver.
A Best User Selection (BUS) scheduler is a typical scheduling technique used to obtain a multiuser diversity gain in the conventional antenna array system environment supporting multiple users. The BUS scheduler transmits data to a user having the maximum channel capacity in each scheduling interval. If the total number of users is denoted by K, the BUS scheduler allocates resource kBUS*(t) to each individual user in accordance with Equation (1).
                                          k            BUS            *                    ⁡                      (            t            )                          =                                            arg              ⁢                                                          ⁢              max                                                      k                =                1                            ,              …              ⁢                                                          ,                                                          ⁢              K                                ⁢                                    C              k                        ⁡                          (              t              )                                                          (        1        )            where Ck(t) denotes channel capacity for a user k at a time t.
FIG. 1 is a diagram illustrating a configuration of an antenna array system for allocating resources to individual users using a conventional BUS scheduler.
Referring to FIG. 1, users User 1 120, User 2 122, . . . , User K 124 send to a base station channel state information Ck(t) (k=1, . . . , K) determined by taking channel characteristics into account. The channel state information Ck(t) (k=1, . . . , K) for the individual users is provided to a packet scheduler 110 in the base station. The packet scheduler 110 determines resource kBUS*(t) to be allocated to each individual user in accordance with Equation (1) by taking into account the channel state information Ck(t) (k=1, . . . , K) for the individual users.
The resource kBUS*(t) allocated to the individual user is provided to a buffer 112. The buffer 112 is a queue for temporarily storing the packets to be transmitted to individual users. The buffer 112 outputs the stored transmission packets for the individual users with the resources kBUS*(t) allocated to the individual users. The transmission packets output from the buffer 112 are provided to a coding and adaptive modulation unit 114. The packets to be transmitted to the individual users undergo coding and modulation based on a specific modulation scheme in the coding and adaptive modulation unit 114. The modulation scheme is adaptively changed according to the channel state information provided for the individual users.
The modulation symbols to be transmitted to the individual users are multiplexed by a spatial multiplexer 116 with modulation symbol streams, the number of which is equal to the number of transmission antennas. The modulation symbol streams multiplexed by the spatial multiplexer 116 are transmitted to the individual users via their associated transmission antennas.
The multiantenna system supporting multiple users, shown in FIG. 1, has unique channel characteristics H1(t), H2(t), . . . , HK(t) between the base station and the users. The users 120, 122 and 124 determine channel state information Ck(t) (k=1, . . . , K) taking their own channel characteristics into account, and feed back to the base station the channel state information.
The foregoing conventional BUS scheduler allocates more resources to a user having higher average channel capacity when there is a difference in average channel capacity between users, so fair resource allocation cannot be achieved for the respective users. That is, the user having a bad average channel state can be allocated almost no resources.